1. Field of the Invention
The present invention relates to a photoreceiver circuit and more particularly, to a photoreceiver or photoreceptor circuit equipped with a photoelectric conversion element and an amplifier circuit, which is capable of conversion-gain adjustment of the photoelectric conversion element and high-speed circuit operation. For example, this photoreceiver circuit is applicable to intelligent sensors for sensing a moving object or objects in an image formed by a photoelectric conversion element (i.e., a scene).
2. Description of the Prior Art
An example of the prior-art photoreceiver or photoreceptor circuits each having photoelectric conversion elements and amplifier circuits is disclosed in the U.S. Pat. No. 5,376,813 issued on Dec. 27, 1994, which is intended to expand the dynamic range with respect to the incident light, resulting in increase in response speed. The circuit configuration of this prior-art photoreceiver circuit thus patented is shown in FIG. 1.
In FIG. 1, a photodiode 301 serves as a photoelectric conversion element. One terminal of the photodiode 301 is connected to the gate of an n-channel Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) 302. The other terminal of the photodiode 301 is connected to the ground. The source of the MOSFET 302 is connected to the ground. The drain of the MOSFET 302 is connected to the drain of a p-channel MOSFET 303. The source of the MOSFET 303 is connected to a power supply (not shown) supplying a constant supply voltage V.sub.cc. The gate of the MOSFET 303 is applied with a suitable bias voltage V.sub.bias.
The combination of the MOSFETs 302 and 303 serves as an inverting, analog voltage amplifier circuit 310 for amplifying a voltage V.sub.a at the output terminal 300A of the photodiode 301 with respect to the ground (i.e., an input voltage V.sub.in of the amplifier 310). The MOSFET 302 is operated in the saturation region. The MOSFET 303 serves as a load resistor of the MOSFET 302 in the amplifier circuit 310.
An output voltage V.sub.out of the voltage amplifier circuit 310, which is an amplified voltage of the input voltage V.sub.in, is derived from the drain of the MOSEET 302 or an output terminal 300B. The output voltage V.sub.out is fed back to the input side of the amplifier circuit 310 through a voltage-lowering circuit 330 and an n-channel MOSFET 307. The voltage-lowering circuit 330 comprises two capacitors 304 and 305. The capacitor 304, which as a capacitance C.sub.1, is connected to the output terminal 300B and a terminal 300D connected to the gate of the MOSFET 307. The capacitor 305, which has a capacitance C.sub.2, is connected to the terminal 300D and the ground.
A current-leaking circuit 320, which comprises a p-channel MOSFET 306, is connected in parallel to the voltage-lowering circuit 330 between the terminals 300B and 300D. The gate and the drain of the MOSFET 306 are coupled together to be connected to the terminal 300B. The source of the MOSFET 306, which is connected to the substrate, is connected to the terminal 300D.
The source of the MOSFET 307 is connected to the terminal of the photodiode 301 at the terminal 300A. The drain of the MOSFET 307 is connected to the power supply and applied with the supply voltage V.sub.cc.
The current leaking circuit 320 serves to leak a current between the terminals 300B and 300D. Specifically, when a potential difference occurs between the terminals 300B and 300D, a current flows gradually (i.e., leaks) through the MOSFET 306 from the terminal 300B to the terminal 300D and vice versa, thereby eliminating the potential difference after a specific relaxation time.
Next, the operation of the prior-art photoreceiver circuit shown in FIG. 1 is explained below.
When the incident light PH applied to the photodiode 301 has a constant intensity with time, i.e., the photodiode 301 is in the steady state, the electric potentials or voltages at the terminals 300B and 300D with respect to the ground are equal to each other because of the current-leaking operation of the MOSFET 306. On the other hand, a voltage V.sub.d at the terminal 300D (i.e., the gate voltage of the MOSFET 307) with respect to the ground is determined in such a way that a current flowing through the MOSFET 307 is equal to an output current I.sub.PH of the photodiode 301.
Thus, the output voltage V.sub.out of the prior-art photoreceiver circuit of FIG. 1 produced at the output terminal 300B is equal to the voltage V.sub.d at the terminal 300D, i.e., V.sub.out =V.sub.d, when the photodiode 301 is in the steady state.
On the other hand, when the intensity of the incident light PH applied to the photodiode 301 varies with time, i.e , the photodiode 301 is in the changing state, the magnitude of the output current I.sub.PH of the photodiode 301 varies with time according to the intensity change of the light PH, thereby changing the magnitude of the voltage V.sub.a at the terminal 300A. The change of the voltage V.sub.a at the terminal 300A is applied to the amplifier circuit 310 as its input voltage V.sub.in and is amplified therein, producing an amplified change of the output voltage V.sub.out at the terminal 300B. This amplified change of V.sub.out is opposite in phase to the change of V.sub.a and therefore, the latter is decreased if the former is increased, and vice versa. The amplified change of the output voltage V.sub.out is sent to the gate of the MOSFET 307 through the voltage-lowering circuit 330, causing an amplified change of the current flowing through the MOSFET 307. Thus, the current flowing through the MOSFET 307 is equalized with the output current I.sub.PH of the photodiode 301.
As explained above, the output voltage V.sub.out of the photoreceiver circuit is fed back to the input side of the amplifier circuit 310 through the voltage-lowering circuit 330 and the n-channel MOSFETs 307, thereby suppressing the change of the voltage V.sub.a at the terminal 300A caused by the change of the output current I.sub.PH. As a result, the value of the voltage V.sub.a is kept approximately constant independent of the intensity change of the incident light PH.
The above-described circuit operation of the prior-art photoreceiver circuit of FIG. 1 is unlike that of another prior-art photoreceiver circuit shown in FIG. 2. The circuit in FIG. 2 is simply comprised of a photodiode 401 and an n-channel MOSFET 402 without any feedback path. An output terminal 400A of the photodiode 401 is connected to the source of the MOSFET 402. The gate of the MOSFET 401 is applied with a fixed bias voltage V.sub.b. An output voltage V.sub.out of the photoreceiver circuit is derived from the output terminal 400A.
In the circuit of FIG. 2, since no feedback path is provided, the output voltage V.sub.out produced at the terminal 400A varies largely in order to equalize the current flowing through the MOSFET 402 with the output current I.sub.PH of the photodiode 401. This is quite different from that of the prior-art photoreceiver circuit shown in FIG. 1 where the current flowing through the MOSFET 307 is equalized with the output current I.sub.PH of the photodiode 301 by changing the gate voltage V.sub.d of the MOSFET 307.
In the circuit of FIG. 2, the parasitic capacitors existing in the vicinity of the terminal 400A (e.g., the parasitic capacitors of the photodiode 401 and the source region of the MOSFET 402) need to be charged and discharged by the output current I.sub.PH itself of the photodiode 401. Since the output current I.sub.PH is usually very small, it takes a long time to fully charge or discharge these parasitic capacitors. This means that the necessitated relaxation time of the photoreceiver circuit of FIG. 2 from the changing state to the steady state is extremely long.
In contrast, in the photoreceiver circuit of FIG. 1, the voltage V.sub.a at the terminal 300A is always kept approximately constant because of the operation of the MOSFET 307. Thus, the parasitic capacitances need not be charged nor discharged, which shortens the relaxation time. This creates an advantage of high-speed circuit operation.
In addition to the advantage of high-speed circuit operation, the prior-art photoreceiver circuit of FIG. 1 has another advantage that the gain of the photoreceiver circuit in the changing state is different from that in the steady state. Here, the term "gain" means the coefficient of photoelectric conversion in this photoreceiver circuit, in other words, it means the ratio of the magnitude of the output voltage V.sub.out to the intensity of the incident light PH. Because of this variable gain, the prior-art photoreceiver circuit of FIG. 1 has a wider dynamic range than that of the prior-art photoreceiver circuit of FIG. 2.
Subsequently, adjustment of the gain in the photoreceiver circuit of FIG. 1 is explained below.
As explained previously, the p-channel MOSFET 306 of the current-leaking circuit 320 has a function to leak the current between the terminals 300B and 300D. Thus, if the intensity of the incident light PH varies abruptly and therefore, the output voltage V.sub.out generated at the terminal 300B is abruptly changed, the voltage change at the terminal 300B is transmitted to the terminal 300D through the capacitors 304 and 305 of the voltage-lowering circuit 330. In this case, the voltage V.sub.d at the terminal 300D is equal to [C.sub.1 /(C.sub.1 +C.sub.2)] times the output voltage V.sub.out because it is divided by the capacitors 304 and 305, where V.sub.d &lt;V.sub.out. As a result, to equalize the current flowing through the MOSFET 307 with the output current IPH of the photodiode 401, the output voltage V.sub.out at the terminal 300B needs to be higher than a voltage required in the steady state. This means that the gain of the photoreceiver circuit of FIG. 1 in Lhe changing state is higher than that in the steady state.
However, the state where the voltage V.sub.d at the terminal 300D is equal to [C.sub.1 /(C.sub.1 +C.sub.2)] V.sub.out occurs only in the changing state where the intensity of the incident light PH varies. After the intensity change of the light PH disappears and the circuit operation enters the steady state, the MOSFET 306 allows the current to leak from the terminal 300B to the terminal 300D and vice versa, resulting in the output voltage V.sub.out at the terminal 300B being equal to the voltage V.sub.d at the terminal 300D.
In other words, in the changing state, the feedback loop is constituted by the capacitors 304 and 305 and the MOSFET 307 and therefore, the divided voltage V.sub.d is applied to the gate of the MOSFET 307. Unlike this, in the steady state, the feedback loop is constituted by the p-channel MOSFET 306 and the MOSFET 307 and therefore, the output voltage V.sub.out at the terminal 300B is directly fed back to the gate of the MOSFET 307.
As explained above, in the prior-art photoreceiver circuit of FIG. 1, the value of the photoelectric-conversion gain in the changing state is [(C.sub.1 +C.sub.2)/C.sub.1 ] times as much as that in the steady state, resulting in a wider dynamic range than that in the circuit of FIG. 2.
With the prior-art photoreceiver circuit of FIG. 1, however, there is a problem that the gain value of the photoreceiver circuit is unable to be optionally adjusted from the outside. This problem occurs not only in the changing state where the intensity of the incident light PH varies (i.e., the capacitors 304 and 305 constitute the feedback path) but also in the steady state where the intensity of the light PH does not vary (i.e., the MOSFET 306 constitutes the feedback path).
The above problem is caused by the difficulty in gain adjustment of the analog voltage amplifier circuit 310. The adjustment of the gain of the amplifier circuit 310 can be realized only by changing the value of the bias voltage V.sub.bias applied to the gate of the MOSFET 203. However, the change of the bias voltage V.sub.bias is unable to be realized in practice, because the operation of the amplifier circuit 310 is extremely sensitive to the change of the bias voltage V.sub.bias. Accordingly, the bias voltage V.sub.bias is usually fixed at a specific value and is unable to be changed from the outside.
In practice, even if the value of the bias voltage V.sub.bias is changed within an extremely small range, the operating point of the amplifier circuit 310 readily deviates from its optimum point, resulting in abrupt decrease in gain. This means that the amplifier circuit 310 does not provide the desired amplification operation any more.
Thus, although the gain value of the amplifier circuit 310 can be adjusted from the outside by changing the bias voltage V.sub.bias in theory, it is extremely difficult or impossible to be realized in practice.